count number of rows in a data frame in R based on group [duplicate]

Question

The count() function in plyr does what you want:

library(plyr)

count(mydf, "MONTH-YEAR")

Here's an example that shows how table(.) (or, more closely matching your desired output, data.frame(table(.)) does what it sounds like you are asking for.

Note also how to share reproducible sample data in a way that others can copy and paste into their session.

Here's the (reproducible) sample data:

mydf <- structure(list(ID = c(110L, 111L, 121L, 131L, 141L), 
                       MONTH.YEAR = c("JAN. 2012", "JAN. 2012", 
                                      "FEB. 2012", "FEB. 2012", 
                                      "MAR. 2012"), 
                       VALUE = c(1000L, 2000L, 3000L, 4000L, 5000L)), 
                  .Names = c("ID", "MONTH.YEAR", "VALUE"), 
                  class = "data.frame", row.names = c(NA, -5L))

mydf
#    ID MONTH.YEAR VALUE
# 1 110  JAN. 2012  1000
# 2 111  JAN. 2012  2000
# 3 121  FEB. 2012  3000
# 4 131  FEB. 2012  4000
# 5 141  MAR. 2012  5000

Here's the calculation of the number of rows per group, in two output display formats:

table(mydf$MONTH.YEAR)
# 
# FEB. 2012 JAN. 2012 MAR. 2012 
#         2         2         1

data.frame(table(mydf$MONTH.YEAR))
#        Var1 Freq
# 1 FEB. 2012    2
# 2 JAN. 2012    2
# 3 MAR. 2012    1

Using the example data set that Ananda dummied up, here's an example using aggregate(), which is part of core R. aggregate() just needs something to count as function of the different values of MONTH-YEAR. In this case, I used VALUE as the thing to count:

aggregate(cbind(count = VALUE) ~ MONTH.YEAR, 
          data = mydf, 
          FUN = function(x){NROW(x)})

which gives you..

  MONTH.YEAR count
1  FEB. 2012     2
2  JAN. 2012     2
3  MAR. 2012     1

Try using the count function in dplyr:

library(dplyr)
dat1_frame %>% 
    count(MONTH.YEAR)

I am not sure how you got MONTH-YEAR as a variable name. My R version does not allow for such a variable name, so I replaced it with MONTH.YEAR.

As a side note, the mistake in your code was that dat1_frame %.% group_by(MONTH-YEAR) without a summarise function returns the original data frame without any modifications. So, you want to use

dat1_frame %>%
    group_by(MONTH.YEAR) %>%
    summarise(count=n())